Volume 119, Nº 4 (2016)

Aspects of the nuclear safety of the spent fuel pools of RMBK reactors under normal conditions and design basis and beyond design basis accidents are studied using a three-dimensional model verified by comparing with Monte Carlo calculations. It is shown that the storage of spent fuel assemblies in pencil-shaped cases secures nuclear safety in the entire range of water density, lattice spacing, and fuel burnup, i.e., in all, including hypothetical, emergency situations. For compact case-free storage, nuclear safety is secured in real scenarios where the water density changes upon disruption of heat removal or flooding of a dewatered pool. The disruption of the lattice spacing of compact storage in design basis, including seismic action, and beyond design basis accidents is examined. The conditions under which nuclear safety is secured are shown.

The implementation in the MCU software complex of methods and algorithms for calculating the neutronphysical characteristics in volumes set with the aid of a supplementary geometry, which is not related with the main geometry and determines exclusively the regions of calculation of functionals and not the construction of the system being modeled, is described. Three types of supplementary geometry for Monte Carlo modeling of particle trajectories in hexagonal, cylindrical, and rectangular detection networks are implemented. These method and algorithms have greatly simplified the process of setting the initial data, increased the reliability of the calculations and the universality of the code in solving complex problems such as the calculation of the spatial distribution of the energy release and the flux density in models of power reactors.

Continuous monitoring and diagnostics of the core state make it possible to simultaneously analyze and adjust the operation of the nuclear reactor in the event of trouble. The ECRAN 3D system was developed for timely monitoring and diagnostics of the core state and the RBMK-1000 data base. This system makes it possible to discover failures and disruptions in the operation of the reactor xxx installation which are not recorded by the standard monitoring system. The failures could be associated with technical trouble as well as errors in entering the parameters of the reactor installation into the informational-measurement system. The algorithm of the ECRAN 3D system, the results of testing the system on real data on NPP, and the particularities of the interface are described.

A method is proposed for determining the mass and isotopic composition of the plutonium warehoused in containers at the RT-1 plant of the Industrial Association Mayak. A single measurement system with a detector based on ultrapure germanium is used for the measurements. The mass is determined according to the pulse count rate in ²³⁹Pu peaks. The correction for self-absorption of radiation in a container is determined by measuring the transmission of radiation emitted from an external source and by Monte Carlo calculations. Estimates show that the total error in determining the plutonium mass by the new method is 11% (P = 0.95). The method can be used to perform confirmatory measurements in the accounting and control system for nuclear materials.

Computational validation is presented for previous benchmark experiments designed to determine the energy spectra of the neutron current emanating from the surface of a uranium sphere into which small sources were arranged successively at the center. Such experiments must meet stringent reliability and error requirements, since they are intended for the final check of new or adjusted systems of neutron constants recommended for the calculation of neutron fields. The comprehensiveness of the validation was secured by the fact that experiments with the three sources ²⁵²Cf, Pu–Be, and Pu–B were validated and the calculations were performed using three systems of neutron constants BNAB-78, BBF, and BUGLE-96.

Variants of a hypothetical accident that includes the occurrence of a spontaneous chain reaction are examined. The pathways and intensity of radionuclide emission from a nuclear submarine into seawater are determined. The ¹³⁷Cs accumulation at the time of sinking is determined more accurately. The average and maximum ¹³⁷Cs emission for use in predicting the contamination of the marine environment in application to the object under study are determined.